The present invention relates to a method for measuring the flow speed and the gas volume proportion of a stream of liquid metal and to an apparatus for practicing the method.
Such methods and apparatuses gain in significance as the use of liquid metals such as, for example, sodium, as a coolant for nuclear reactors is increased since it is necessary to measure the speed or throughput, respectively, at many components in the system. In particular, it is necessary, in order to avoid local overheating in the area of the fuel elements, for example, as a result of partial blockages, to continuously monitor the stream of liquid metal.
It is also important to detect sufficiently early in time any damage to a fuel rod which becomes evident by a quantity of fission gas being blown into the coolant.
It is known to meet these requirements by arranging the pipeline containing the stream of liquid metal whose speed is to be measured between the pole pieces of a permanent magnet, and to measure the voltage which is induced when a conductor is moved perpendicular to a magnetic field as defined in Faraday's law of induction. (Siemens Magazine Volume 48 (1974), pages 614-617.) Such measuring devices, however, require calibration with temperature dependent characteristics which can be determined only in complicated systems, usually only at the manufacturer's plant. Since the magnetic flux density is included in the measuring result and the field of the permanent magnets changes with age, recalibrations are required which are very complicated to accomplish in the installed state and are often impossible to realize (EUR 1631 d, 1964).
It is further known to perform two-phase current measurements in electrically conducting liquids using Chen probes (see The Review of Scientific Instruments, Vol. 39 (1968) pages 1710-1713), comprising two jacketed thermo-elements which are introduced into the medium to be controlled and whose sensors are welded to the encasing jacket at the point of measurement. The free ends of one sensor are connected with the poles of a stabilized direct voltage source. Changes in conductivity in the region of the Chen probes can be measured as changes in the voltage across the free ends of the other sensor.
However, with such probes it is possible to effect only local measurements within a closely limited region. In order to perform integral measurements, for example, in the plane of an annular chamber, it is necessary to install a plurality of probes over the periphery of the annular chamber. The measuring result is falsified because small gas bubbles flow around the probes and only larger bubbles produce a change in voltage. There is the further drawback that the flow geometry is interfered with by the probes which protrude into the stream and that it is necessary to have available a stabilized external voltage supply.